Storage tanks with floating roofs are widely used to store liquid petroleum products. With such storage there are numerous potential problems which include but are not limited to:    (a) damage to the floating roof from excessive accumulated rain water atop the roof,    (b) escape of flammable and/or pollution-causing vapors upward through an emergency drainage duct for said accumulated rain water, and    (c) escape of said flammable and/or pollution causing vapors upward through vertically extendable support legs provided to support said floating roof in selected elevations. The present invention focuses on the third of the above-listed potential problems, namely escape of vapors through the support legs. At certain times liquid petroleum is removed from below a floating roof so that maintenance persons can access pipes and apparatus at the lower portion of the tank below the roof. The support legs for floating roofs may have various constructions, and the present invention pertains to known support legs where each is formed of a tubular column that is telescopically extendable, and where dangerous vapors may escape through the legs while stationary or while being extended between their low and high positions.
The low position of typically one-to-three feet above the tank bottom, is needed in many tanks for the floating roof to be clear of the pipes and other apparatus located near the bottom of the tank. The high position which might be about seven feet above the bottom is to provide headroom for workers to move about, clean and repair the interior of the tank. Multiple heights are necessary, because if the support columns provided only a single relatively high elevation, there could be an accumulation of vapor in the tank below the roof when the liquid level falls below the high position. Thus, these support columns are axially extendable, and conventional support columns have construction where vapors can escape upward through the columns.
A variety of known methods have been employed to seal the openings where vapor escapes through these support legs, but such methods are unsatisfactory because they are expensive, unsightly, and/or prone to deteriorate when exposed to some tank vapors or sunlight.
A conventional type of seal is described in the two prior art patents discussed below. U.S. Pat. No. 5,305,905 discloses a floating roof with a downward extending leg to support the roof at a selected elevation if the volatile fluid is removed from beneath such floating roof. This patent proposes to block volatile fumes from escaping upward through the annular space between the inner support leg and the outer sleeve secured to the floating roof, by simply employing a blind flange and a gasket across the top of the annular space. To support the floating roof at different elevations, one must replace the first leg with a larger one. This disclosure does not address the problem of leakage of volatile fumes when an inner leg is extendible upward through and beyond the top of the outer sleeve that is secured to the floating roof.
Prior art U.S. Pat. No. 5,353,941 discloses another method for reducing vapor loss through the annular space of a support leg; however, the proposed solution is essentially to have the inner pipe of the support leg pass through a hole in a seal which applies a wiping effect.
Still additional examples of prior art wiping gaskets are seen in U.S. Pat. Nos. 4,243,151, 5,829,621, 3,409,165 and 3,583,594.
A typical prior art support leg for use with a floating roof as described above, comprises a central leg and an outer sleeve or pipe slidable on said central leg and fixable at different elevations by a pin extending transversely through holes in the walls of the sleeve and the central leg. In one arrangement there are a series of holes extending through the opposite walls of the leg at different elevations, and at least one set of holes extending through opposite walls of the sleeve, for coupling the sleeve with the central leg at said different elevations. While the top and bottom ends of the leg may be sealed closed, these holes in the walls of the sleeve and the leg represent potential avenues for volatile liquid evaporated from the tank to escape into the upper region of the tank and/or outside of the tank.
The vapor that escapes through said holes in the walls of the leg, originates from the liquid in the tank and rises upward through the annular space between said sleeve and central leg. Such annular space begins at the bottom end of the sleeve which is necessarily open so that it can slide upward or downward lengthwise on said leg to selected elevations on said leg.
The support leg vapor seals of this invention are intended to reduce or preferably prevent escape of vapors from volatile liquid contained in a storage tank with a floating roof. In particular, the invention is to prevent or reduce escape of said vapors from the floating roof support legs.
A typical support column includes as the support leg preferably round cylindrical tube or inner pipe, and an outer pipe or sleeve of shorter length slidably situated concentrically outward of said inner pipe, with certain annular clearance between the inner pipe and sleeve to allow for free sliding therebetween. The outer pipe is secured to the floating roof, so that when the floating roof descends or liquid is pumped out of the tank to be inspected, cleaned or maintained, the roof will be barred from descending lower than allowed by the support legs extending below the roof.